(1) Field of the Invention
The present invention relates to automatic identification of a PABX call to a switching center and more particularly to circuit apparatus and interconnections for routining and localizing faults in asynchronous clock circuits which control transmission and reception of data sent from a PABX to a telephone central office via a data link.
(2) Description of the Prior Art
Telephone switching centers are connected to PABX's, located on subscriber premises, via trunk circuits. Many individual stations are connected to the PABX. A relatively smaller number of trunk circuits connect the PABX to the switching center. Therefore, each PABX station must dial an access code digit to seize control of an available trunk circuit.
As a result of this oeration, the switching center is unable to distinguish the identity of the particular station originating the telephone call. Historically, operators were utilized to ask the particular station user for his station identity before connecting the call. Such information was manually recorded by the operator.
With the advent of electronics, sophisticated systems for the transmission of this station information from the PABX to the central office were developed. This equipment consisted of electronics located on the PABX subscribers premises, data link equipment connecting the subscriber premises to the switching center, and additional electronics added to the switching center. These systems are termed automatic identification of outward dialing systems. Such systems provide the switching center with the identity of the calling station automatically and without the need of operator intervention.
When a PABX station user dials the access code for a trunk circuit connecting him to the switching center (central office trunk), the station identity is noted at the PABX. Also noted is the identity of the trunk circuit selected, connecting the station through the PABX to the switching center. Typically, these two pieces of information are combined into a 41-bit transmission. The station identity consists of 20-bits, the trunk identity 20-bits and a 1-bit synchronization mark. The above information is transmitted via a separate data link facility to the switching center where it is placed in a temporary storage buffer.
As the central office trunk is seized at the PABX, it causes a "Call-For-Service" to be generated at the switching center. When the "Call-For-Service" is recognized, the switching center identifies the requestor by the trunk identity stored in the center's data base. At convenient points in servicing the call, the temporary storage buffer is searched using the trunk identity obtained when the "Call-For-Service" was recognized. Upon finding a trunk identity concurrence, the station identity is placed into the switching centers memory corresponding to the call. Using the above identified trunk, a billing record is generated including the particular station identity.
The collected information is stored on a suitable output device and interrupted by an electronic data processing center. The processing center is able to generate a detailed billing document containing the charges and the number of calls made by each station within the PABX. These detailed billings aid the corporate customer in accounting for its telephone charges.
Clock circuits form an integral part of such data reception systems and are well known. These circuits provide the useful function of driving gating and logic circuits in the transmission and reception of data. Since other logic circuits depend for their operation on the operation of such clock circuits, the functional operability and dependability of clock circuits is of paramount importance to the required continuous operation of a communication system.
Clock circuits are of particular importance in situations wherein revenues are charged telephone subscribers depending upon the integrity of the information which is processed by logic circuits controlled by the clock circuit. The present block circuit is one which is a portion of a revenue producing system. This system is concerned with the automatic identification of outward dialing (AIOD) of a PABX station. Information concerning the particular station dialing an outgoing call from the PABX is transmitted by this system to produce billing information for each station associated with the PABX.
Therefore, such clock circuits must be highly reliable and maintainable. One solution to this problem is presented in U.S. Pat. No. 3,803,568 issued on Apr. 9, 1974, To C. S. Higashide. This patent teaches the use of two identical clock circuits wired for redundant operation. One clock circuit functions as the main system clock and the other functions as a standby system clock. If a fault is detected, the main system clock pulses are inhibited and the standby system clock pulses are enabled. The clock circuit described therein is of necessity one including synchronous operation of the two clock circuits. Furthermore, these two clock circuits are dedicated to the same functional operation. Such an arrangement is both complex and costly.
Accordingly, it is an object of the present invention to provide a simplified routinable clock circuit utilizing two independent asynchronous system clocks which provide the timing required for the processing of two independent functions. Furthermore, it is an additional object of the present invention to provide a highly reliable clock circuit without need of duplication.